Gasoline is a complex mixture of hydrocarbons generally having 4-12 carbon atoms and a boiling point in the range of about 35-200° C. It is a blend of multiple refinery streams, which fulfill certain specifications dictated by both performance requirements and government regulations. Typical gasoline blending streams, which usually include octane booster additives (oxygenate), such as methyl tert-butyl ether (MTBE) or tetra-ethyl lead, are presented in Table I.
TABLE ITypical Gasoline Blending ComponentsBlending ComponentGasoline (vol %)FCC Gasoline30-50has ~30 vol&(naphtha)aromatics and 20-30vol % olefinsLSR Gasoline2-5(naphtha)Alkylate10-15Oxtane booster10-15additive (oxygenatessuch as MTBE)Butanes<5Reformate20-40has 60-65 vol %aromaticsIsomerate (C5/C6) 5-10
Generally, FCC naphtha and reformate make up approximately two-third of gasoline. Since FCC naphtha and reformate contain high levels of aromatics and olefins, they are also the major octane sources for gasoline.
FIG. 1 represents a simplified perspective view of a process diagram according to an embodiment of the prior art. Naphtha feed 2 is introduced into first separator 10, where it is then split into light naphtha 12 and heavy naphtha 14. Light naphtha 12 generally contains mostly C5 and C6 paraffins. Light naphtha 12 is then introduced into first isomerization unit 20 in order to isomerize light naphtha 12 to form light isomerate 22. Heavy naphtha 14 enters reforming unit 30, where heavy naphtha 14 is reformed to reformate 32. Light isomerate 22 and reformate 32 are then blended together in gasoline blender 40 to form gasoline blend 42.
Over the years, safety and environmental concerns have caused gasoline specifications to change. For example, European gasoline specifications from 1995 to 2005 are presented in Table-2, which shows a gradual change of the gasoline specifications over the years. A similar trend is also observed in the other parts of the world.
TABLE IIEuropean Commission Gasoline SpecificationsParameter1995200020052005+Octane number; RON—959595Aromatic, vol %—4235<35Benzene, vol %511<1Sulfur, ppmw100015050/10<10Olefins, vol %—181810Oxygen, wt % max2.72.72.7—Rvp, psi—8.78.78.7
Table II also shows that there is a gradual decrease in aromatic, olefin, and benzene levels while keeping high octane value. The United States already requires aromatic levels of less than 30 vol %, with benzene levels being limited to 0.8%. Furthermore, the aromatic level in gasoline will also be lowered, particularly as distillation end points (usually characterized as the 90% distillation temperature) are lowered since the high boiling point portion of gasoline (which is largely aromatic) would thereby be eliminated. Furthermore, since aromatics are the principle source of octane, decreasing aromatics level will create an octane gap in the gasoline pool. As such, octane-barrel maintenance will continue to be a challenge for refineries.
As aromatic content of gasoline goes down, the portion of reformate in the gasoline poll has to go down accordingly since reformate is mostly aromatics. Therefore, refineries can no longer heavily rely on aromatics as octane source. An ecologically sound way to increase the octane number is by increasing the concentration of the branched alkanes at the expense of normal paraffins. Consequently, an increase in iso-alkanes with high octane number is desirable.
It would be desirable to have an improved process for refining naphtha that resulted in an improved gasoline blending streams and/or to produce concentrated reformate for petrochemicals.